The present invention relates to an inkjet recording apparatus that expels ink continuously from a nozzle and prints characters or patterns on articles that are transferred on a production line.
Inkjet recording apparatus based on the continuous method, in which ink is expelled from a nozzle continuously, ink droplets being moving in the air are charged, and an electric field is used to deflect the ink droplets for printing, are widely used to print numerals and symbols on metal cans and plastic surfaces. Conventional inkjet recording apparatus comprise a main body, a recording head, and a cable for interconnecting the main body and the recording head, as disclosed in Japanese Application Patent Laid-Open Publication Nos. 2000-203050 and 2001-138544. The main body has an ink container for storing ink, a pump for supplying the ink from the ink container to the recording head, another pump for collecting ink from the recording head into the ink container, and a control section for controlling the operation of the recording apparatus. The recording head has a nozzle for expelling the ink supplied from the main body as ink particles, charging electrodes for charging the ink particles, deflecting electrodes for deflecting the charged ink particles by means of an electric field, and a gutter for collecting ink that has not been used. The cable for interconnecting the main body and the recording head includes a tube through which ink flows and electric wires that transmit electric signals to the recording head.
To collect unused ink particles from the gutter, the atmospheric pressure around the gutter heeds to be negative, but the atmospheric pressure can be reduced only down to zero. In the structures of the conventional inkjet recording apparatus, the gutter is open to the atmosphere, so the maximum possible differential pressure produced by the ink collecting pump between the gutter and the ink collecting pump is equal to the atmospheric pressure. Accordingly, the maximum length of the ink collecting flow path between the main body of the inkjet recording apparatus and the recording head has to be limited to a length for which the differential pressure equal to the atmospheric pressure is enough to collect ink from the gutter to the main body equipped with the ink container. This has been an obstacle to flexible adaptation to user equipment.
Another problem with the conventional inkjet recording apparatus described above is that the pump for collecting ink not used in the recording head into the ink container is disposed in the main body. The inkjet recording apparatus related to the present invention is intended for use with a production line, so the length of the tube for interconnecting the main body and the recording head is generally preset in the range from about 2 m to 4 m. Therefore, the pump for collecting ink not used in the recording head into the main body must have a capacity enough to collect ink from a position 2 to 4 m apart. In this case, ink and air are collected together. When a pump is used to transfer a mixture of a liquid and a gas, if the pump is positioned near the transfer source, stable transfer with less flow rate variations in time can be achieved, as compared when the pump is positioned near the transfer destination. In the description that follows, the flow rate indicates the flow rate of a mixture of a liquid and a gas.
The conventional inkjet recording apparatus collects ink from a distant position, which needs a high flow rate so that ink can be collected stably even when the flow rate varies to a low value. As the temperature of the ink falls, its viscosity increases, thereby increasing the flow path resistance generated when the ink flows through the tube. If the recording head is positioned below the main body, the flow path resistance during collection becomes large. To collect ink stably even in a situation in which the flow path resistance during collection becomes large as described above, it is necessary to use a pump that has a high collection flow rate. The ink used by the inkjet recording apparatus related to the present invention needs to be dried quickly after printing, so methyl ethyl ketone (MEK) or another highly volatile substance is used as the solvent. When the collection flow rate is set to a large value, much air is sucked and the amount of ink solvent vapor increases, adversely affecting the environment. When persons work in a room in which a production line is installed, they also suffer from the adverse effect by the ink solvent vapor.